The primary consolidating fluid used in such pumps is generally a binary fluid consisting of water and cement. This fluid is injected into a hole in the soil to be consolidated through a drilling rod string at the bottom of which there is fixed an injection head, called a “monitor”, which has at its outlet at least one very small diameter nozzle capable of increasing the injection pressure to very high values. It is also a common practice to inject ternary fluids consisting of plastic mixtures of water, cement and bentonite, which are used to make a soil impermeable instead of increasing its mechanical characteristics. Sometimes it is possible to use a pumping system for injecting only one of the fluids described above (e.g. water), in order to treat the soil, to bring about hydraulic disaggregation or for other purposes known in the field. There is also a known practice of combining with the primary fluids particular additives to vary some of their characteristics (setting time, plasticity, consistency, strength etc.).
The range of pressures of such pumps runs from 50 to 1000 bars, while the flow rates vary from a few hundred liters per minute to more than 1000 liters per minute. The cement makes the mixture abrasive, with consequent wear problems for some components of the pump.
For a better understanding of the state of the art and of the problems relating thereto, a description first will be given of a high pressure pump of a known type for injecting cement mixtures (primary fluid), making reference to FIG. 3 in the attached drawings.
The pump makes use of three single-action suction and force plungers such as the one indicated by reference number 11. The plunger is sealingly supported and guided in its reciprocating motion by a sealing device 20, which includes a cylindrical sleeve 21 locked by means of a clamping ring 22 coaxially inside a flanged supporting bush 23. A closed circuit is formed in the sealing device for a second lubricating fluid (or secondary fluid), in particular lubricating oil, with two ducts, inlet 24 and outlet 25, formed in the inner sleeve and in the bush, and an axially extended annular chamber 26 formed in the internal cylindrical cavity 27 of the sleeve, around the plunger. At the two opposite sides of the lubrication chamber 26 a respective annular oil sealing gasket 28, 29 is provided, fixed to the cylindrical sleeve 21 and acting against the plunger. At the end of the sleeve on the “wet” side facing towards the pumping chamber, there is fitted in the internal cylindrical cavity a sealing gasket 32 sealing against the primary fluid, particularly cement; at the opposite end, on the dry side near the clamping ring 22, a scraper ring 33 is mounted.
Currently, gaskets sealing against cement have an average life of about 200-300 hours, depending on the type of cement and the operating conditions: pressure, flow rate and SPM (number of strikes per minute). There is no device capable of indicating wear on the seals. Failure of the cement gasket to seal causes contamination of the secondary fluid lubricating the plungers. The presence of cement in the lubricating oil indicates that the gaskets are no longer sealing; in these conditions, however, it becomes necessary to replace not only the gaskets but the oil itself, and often overloading problems are created for the pump, the filter and the other components in the circuit. In these conditions the sleeve of the sealing device, too, is subject to premature wear due to an increase in friction with the plunger (no longer guided by the worn gasket) and to the presence of cement in the lubricating oil. The oil must normally be replaced every 500 hours, and thus it would be particularly useful to have gaskets capable of working for at least the same amount of time in order to reduce the frequency and cost of servicing. In fact servicing procedures to replace seals are very complex and require the dismantling of many components. Such servicing can take several hours; if performed simultaneously with replacing the oil, there would be an enormous simplification of the servicing process and costs would be drastically reduced.